Vacuum infusion is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, said fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of single fibres or woven mats made of fibre rovings. The second mould part, which is often made of a resilient vacuum bag, is subsequently placed on top of the fibre material. By generating a vacuum, typically 80 to 90% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained therein. So-called distribution layers and distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum is generated by the vacuum channels in the mould cavity, said vacuum in this connection being understood as a negative pressure, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in the mould cavity as a flow front moves towards the vacuum channels. Thus it is important to position the inlet channels and the vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas, where the fibre material has not been impregnated, and where there can be air pockets, which are difficult to remove by controlling the vacuum pressure and possibly an overpressure at the inlet side. In connection with vacuum infusion employing a solid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by for example perforating the cloth in the respective locations and by sucking out air by means of a syringe needle. Liquid polymer can optionally be injected at the relevant location, which can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mouldings, the staff has to stand on the vacuum bag, which is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus result in local weakenings of the structures.
Patent literature discloses examples of employing a semi-permeable membrane for distributing the vacuum pressure and thus reducing the above problems. In this connection the term semi-permeable membrane is understood as a membrane, which is permeable to gases but impermeable to liquid polymer. Thus if a semi-permeable membrane is placed over the fibre insertion, air pockets can be removed easily.
US 2003/0011094 A1 discloses the method of placing a distribution layer, through which the liquid polymer quickly can disperse, at one side of the fibre insertion and a semi-permeable membrane at the opposite side of the fibre insertion. One disadvantage of this method is that the finished body has an outer distribution layer without any fibres or with only a small amount of fibres, and thus it does not contribute to the bending rigidity of the body to any serious extent.
In connection with producing relatively thick fibre composite mouldings by means of vacuum infusion, it is known to place intermediate distribution layers or flow layers inside the fibre insertion, so that the liquid polymer can flow into the fibre insertion through said distribution layers or flow layers and spread perpendicularly into the fibre material. However, this method can cause problems with regard to dry spots, as it can result in a plurality of converging flow fronts, which can encase air pockets.